2 wrote:
>
>Another consideration is that 2, 8877s have 0.2pF of feedback-C. Since
>the 8877 is capable of oscillating) well into the uhf range, this
>amount of feedback-C is nothing to sneeze at.
Two tubes also have double the input capacitance and half the input
impedance, so the feedback situation remains the same.
> [intermittent vhf/uhf parasites have a reputation for causing
>intermittent hv-to-gnd arcs, so lower vhf-Q suppressors might not be a
>bad idea]
>
If the grid is nailed down firmly - VHF-style, using multiple contact
fingers on the grid ring - then it's questionable whether any VHF
parasitic suppressors are needed in a single-8877 amp for HF. After all,
the same tube is quite stable in VHF amps, where the input and output
are both *deliberately* tuned to the same frequency! The same of course
applies to the 3CX800 and similar ceramic tubes with a full grid ring.
However, with two tubes in parallel there is also the possibility of
push-pull VHF resonances, so parasitic suppressors would probably be a
good idea to kill this mode.
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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